Dispersing carbon fiber in a matrix such as a resin is a widely and commonly performed technique for imparting, for example, electrical conductivity or thermal conductivity. In the case where vapor grown carbon fiber is added to a resin, even with the addition amount relatively small such that moldability of the resultant resin composition and the appearance of a molded product are not adversely affected, the electrical conductivity and thermal conductivity of the resin composition can be greatly improved (Japan Patent No. 2862578, WO91/03057).
In general, in a case where vapor grown carbon fiber is added to a resin or the like, the larger the aspect ratio of the carbon fiber, the higher the property for imparting electrical conductivity and thermal conductivity. A currently commercially available vapor grown carbon fiber (VGCF, registered trademark; product of Showa Denko K. K.) has a mean fiber diameter of about 150 nm, an aspect ratio of about 45 and a fiber length as short as about 6.7 μm.
As an example of carbon fiber having a relatively large fiber length, Japan Patent No. 1701869 (WO86/03455) describes a carbon fiber having an aspect ratio of 100 or more. However, with the small fiber diameter 3.5 to 70 nm of this carbon fiber, the specific surface area is large and the surface energy is high and therefore, the carbon fiber involves problems in that the carbon fiber tends to form an aggregate, that the carbon fiber is difficult to disperse in a resin, and that therefore, a composite material containing the carbon fiber exhibits unsatisfactory characteristics such as electrical conductivity.